Download Species Interactions: Competition

Interspecific Competition
There are many forms of interaction between/among
populations. Before focusing on interspecific competition let’s
quickly review the full spectrum of interactions. One of the
best ways is to consider the impact of interactions on the
component populations:
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recipient pop.
donor population
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mutualism commensalism predation…
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neutralism
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amensalism competition
Of these interactions, intensive research has basically
concentrated on competition and predation/herbivory.
Competition was believed to be the key factor regulating
populations from the early 1960s until ~1980. Why?
1. The ‘stars’ (MacArthur, Connell, May,…) thought so. As in
most fields, it is difficult to reject the dominant view and
still be successful in getting grants and publications.
2. One seminal paper had singular influence over ecological
thinking. That paper is still fondly called “The Etude”. It
was so important, you should think critically about what it
said. It is a paper written in broad-stroke generalities. In the
end we will reject those generalities (the devil is in the
details), but first allow yourself to be swept up by them.
The Etude: Hairston, Smith and Slobodkin (1960) Community
structure, population control, and competition. Am. Nat.
94:421-5.
1. Hairston, et al. first observe that in contemporary
communities fossil fuels are accumulating at a negligible
rate compared to the amount of photosynthesis occurring
world-wide. If decomposing plant material is not
accumulated, then it is being used up. Decomposers, as a
trophic level, are limited by their food resources.
If that's true, then species in the decomposer trophic level
must, on average, be competing for those limited food
resources.
2. Consider the producer (plant) trophic level. From our
earthly point of view plants seem (quoting the paper)
'abundant and largely intact'. Only in exceptional, nonnatural conditions is over grazing or large scale destruction
of the plant community evident. There are natural
occasions, but they are spatially and temporally rare.
Similarly, weather catastrophes do not usually destroy large
areas of plant community. On average, then, the world is
green.
3. If so, then plants are not limited from above by their
herbivores, nor by catastrophic weather, yet we are not
overrun by plants. They must then be limited by resources
(space, light, nutrients) and compete for those resources.
4. If 'the world is green', then herbivores are clearly not limited
by their food resources; those are present in abundance.
Some herbivores may, at some times, be controlled by
weather or the abundance of plant food, but Hairston, et al.
argue that those are exceptional cases. They suggest that
significant herbivore damage to vegetation occurs most
frequently when non-native herbivores are introduced.
Those introduced species are the ones least likely to be
adapted to local weather conditions, and therefore most
likely to be controlled by them. Since non-natives are the
species which most damage plant communities, weather is,
on average, an unacceptable explanation for control. If
neither resources nor weather control herbivores, but they
do not usually increase to plague levels, then they are
controlled, and predators must be the cause.
5. Finally, consider predators (and parasites by association). If
predators successfully control herbivore numbers, then they
limit their own food supply, and must, as food-limited
species, compete for the limiting resource. Territoriality,
most common among predator species, is an adaptation to
protect sufficient resource to ensure survival.
The conclusions of The Etude are, therefore, that of four
trophic levels, 3 are controlled (regulated?) by competition:
decomposers, producers, and predators; only one is not
regulated by competition, the herbivores.
Since then, we’ve learned that the world was not so green as
Hairston, et al. claimed; plants have an enormous variety of
structural and chemical defenses which, more or less
frequently, limit herbivore populations.
We’ve also learned how important predation can be in
structuring communities. Predators active at the time of
flowering and seed maturation can dramatically alter plant
success. Keystone predators alter species composition and
relative abundance of prey communities.
To begin to think about competition as a limiting and/or
regulating factor, recognize that all populations that are
resource limited must undergo intraspecific competition.
We’ll begin there.
1) competition acts as a controlling agent only when its
ultimate effect is to decrease the contribution of competing
individuals to future generations below that which would
have been made had there been no competition, i.e. it must
have impact on fitness.
2. competition only occurs when some resource necessary
to survival and/or reproduction is present only in a
limited supply.
3. in most cases competition is a bi-directional interaction, i.e.
each competing individual has impact on the success of all
with whom it competes, and is mutually affected by its
competitors. The interaction need not, however, be
symmetrical; there can be dominant and subordinate
individuals in an interaction, with unequal effects on each
other.
4. Competition is density dependent, i.e. the probability of an
individual being affected, and/or the intensity of effects of
interaction, increase with the density (or number) of
individuals in the population.
When intraspecific competition occurs, it is seen as taking
two forms, and two types of characterizations have been
developed:
Nicholson, in his studies of sheep blowflies, defined the two
extreme forms. He called those extremes 'scramble' and
'contest‘ competition.
More recently an alternative division separates categories
called 'interference' and 'exploitation‘ competition.
There are similarities between the two categorizations.
Contest/Scramble
Scramble competition assumes a different distribution of
resources than does contest interaction. At low densities all
individuals get sufficient resource to grow at a maximum rate
(i.e. lx and mx are unaffected by interaction).
While rates (growth, survivorship, and fecundity) decline with
increasing density, in scramble competition all individuals
continue to get an equal share of resources as density
increases. Therefore, above some threshold density each
individual gets less resource than it needs to maintain an R0 of
1; individuals do not survive and/or reproduce successfully,
and the population decreases to 0.
Graphically:
In scramble competition it is assumed that the interaction is
symmetrical. On the other hand, in contest competition the
interactions are asymmetrical. Some individuals win in
competitive contests with others. For simplicity, we assume
that the winners each have equal abilities, and similarly the
losers. Losers get no resources above the threshold, but
winners get an equal share of the resource base. Above the
threshold only winners survive, and their number is fixed by
the resource base; all others die.
In scramble competition all individuals produce an optimal mx
below threshold density, but above it, with insufficient
resources, all have a 0 mx.
In contest competition, winners continue to reproduce at
optimal levels, while losers cease reproduction.
Interference/Exploitation
In interference, one part of a population (or one species)
prevents access of the other to a resource, while consuming
only its own needs.
Resources are sequestered by those who are successful at
interference. Like contest competition, the winners function as
if unaffected by competition, while the losers die.
Exploitation’s effects are not absolute at any threshold. In this
case both kinds (populations, species) use resources with
(usually) different efficiencies. The presence of the competitor
lowers the resource use of each species, and therefore its
success, but, unless competitive exclusion occurs or there are
no alternative resources, both persist with increased mortality
and/or decreased fecundity in the short term.
The relative abilities or resource use efficiencies of the species
involved determine how symmetrical or asymmetrical the
interaction is.
If competition is asymmetrical (or in the traditional LotkaVolterra description, 12  21 with equal Ks) then the
equilibrium condition is the presence of only the stronger
competitor).
For example, 2 species of bumblebees are each, when alone,
able to exploit 2 flower species (a larkspur and a monkshood).
The length of their probosces and the length of the corollas on
the two flowers are different. When both bees are present,
each depletes the nectar resources of the flowers on which it is
the more efficient forager to levels which make the resource
effectively useless for the other.
Exploitation interactions produce a competitive exclusion
(Inouye 1978) based on competitive asymmetry with respect
to each of the two resources.
But does competition “regulate” populations?
What does “regulation” mean?
The classic example that shows competition can regulate
population is Nicholson’s laboratory studies of sheep
blowflies…
The adult sheep blowfly, Lucilia cuprina, lays its eggs on
sheep carcasses or meat, and the meat is the food source for
larval development. In addition, the amount of meat
available for a larva determines its egg laying potential.
The adult also feeds on carcass meat. While the results are
the same, Nicholson, in separate experiments:
gave adults surplus food (no competition) but limited food
available to larvae, or
offered larvae unlimited food resources (no competition), but
gave adults a limited food resource.
The same sort of cycles resulted in each case.
Let's follow the results when adults were given surplus food:
when adults have surplus food
large numbers of adults
pupation increases
more food available
per larva
fewer eggs are produced
adult population declines
high egg production
many larvae
food depleted
(limited food available)
proportion successfully
pupating is low
reduced recruitment into
the adult population
This scheme is obviously cyclic. It produces population cycles
with a period of 30-50 days, and the cycling is caused by
'regulation' due to competition among larvae.
A key problems with Nicholson's sorting out of competition
(and the separation into contest and scramble forms of
competition) is the insistence upon threshold density levels
associated with qualitative changes in growth, reproduction,
or survivorship as density passes through that threshold. Real
experiments show little evidence of such a threshold, at least
one leading to qualitative change. Instead, most competition
experiments seem to find gradual increase in interactive
effects as density increases.
Much of the best quantitative data measuring effects of
competition comes from studies of plants. A paper by
Palmbald (1968), for example, expressly deals with
population size results, rather than the more usual measures of
'yield‘ in studies of agricultural plant competition.
Palmbald studied a variety of weeds, most annual, so that seed
output was a measure of population growth. What he found
was differing kinds of response in different species, but a
tendency for the weedy annuals to have plasticity in
reproduction, and perennials to have plasticity in growth
parameters.
Capsella bursa-pastoris (shepherd’s purse) is fairly typical of
annuals. Germination and pre-reproductive mortality show
only slight responses, but the size of individuals and the
number of seeds produced per individual are strongly affected
by density.
Seed production falls dramatically, from 23,000 at 1 plant per
pot to 210 at 200 per pot, that is by a factor of about 100.
Close to parallel is the decline in biomass per plant. Thus, the
allocation of biomass to produce seeds is relatively constant,
measured as the number of seeds per gram individual plant
biomass.
The data follows:
Sowing density
1
% germination
100
% mortality
0
% reproducing
100
% vegetative
0
Dry weight (g)
2.01
Dry weight/plant 2.01
Seeds/repro23741
ducing individual
Total seeds
23741
5
100
0
100
0
3.44
0.68
6102
50
83
1
82
0
4.83
.096
990
100
86
3
83
0
4.51
.045
451
200
83
8
73
2
4.16
.021
210
30509 40311 37196 30074
There are two things going on:
One is measured at the individual level, where growth, size,
and reproduction change approximately in proportion to
density over fairly wide ranges.
The other is reproductive output per unit area. This is the
measure important to agriculture. A farmer wants to maximize
his yield per acre, while minimizing his costs (here seeds
planted). Since seminal papers in the 1950's it has been clear
that there is a maximum production per unit area. Below some
'threshold' density the total production increases
toward that maximum; above it plasticity (an almost universal
characteristic of plant growth and reproduction) reduces the
growth, etc. of individuals and maintains that maximum.
Plants 'compensate' for density changes, and the result is the
'law of constant final yield' (Kira,et al. 1953,Yoda et al. 1958).
Bromus grown in
pots
Corn grown in a
field
Do plants exactly compensate for density?
The answer is no. Plants 'undercompensate' at low densities,
and totals increase. At higher densities they frequently
'overcompensate', i.e. totals do not remain exactly constant,
but decrease slightly. That’s what was evident in the data for
Capsella.
The growth of Bromus follows what would be called
compensation, but seed production at very low plant densities
cannot reach the maximum yield.
The growth of corn shows ‘overcompensation’, in that total
grain yield declines at very high densities.
Also note the impact of different fertilizer levels. The
maximum yield increases in Bromus when additional nitrogen
fertilizer is added. In general, the maximum yield is set by
environmental conditions, both biotic (e.g. weed occurrence
and density) and abiotic (climate, soil nutrients).
Compensation develops over time.
When plants start growth, the biomass density is low, and total
biomass in pots increases with further growth (which could be
described as 'undercompensation' if yield were measured at
each time by the total biomass of plants).
However, as they grow, biomass density increases, until is
approaches the maximum. If we look at yield-density curves
over time, they begin as straight lines, yield increasing in
proportion to density.
At mid-season the relationship maintains the same slope at
low density, but curves over to flatten at the maximum when
plants are at a higher density. Late in the season the curve has
reached its aymptote at all densities, and is flat.